1. Field of the Invention
The present invention relates to a method for forming a carbon nanotube thin film, which is a thin film having carbon nanotubes that are two-dimensionally spread all over the thin film.
2. Description of the Related Art
A carbon nanotube is a new material containing only carbon atoms as the constituting element discovered in 1991. A carbon nanotube has a cylindrical shape having one-dimensionality formed by winding a graphene sheet constituted by six-membered rings of carbon atoms. A carbon nanotube having a structure containing only one graphene sheet is referred to as a single wall carbon nanotube (SWNT), and a carbon nanotube having a multilayer structure is referred to as a multiwall carbon nanotube (MWNT).
An SWNT has a diameter of about 1 nm, and an MWNT has a diameter of several tens nanometers, which are extremely thinner than those materials that have been referred to as carbon fibers. A carbon nanotube has a nature that is greatly different from other materials constituted only by carbon atoms, such as graphite, amorphous carbon and diamond.
A carbon nanotube has multimodal electric characteristics including metallic nature and semiconductor nature depending on the mode of arrangement of atoms (chirality), and is excellent in mechanical strength. Therefore, a carbon nanotube is receiving attention in various fields of art.
In the field of electronics, for example, various attempts have been made to utilize the particular electric capability of a carbon nanotube in a field effect transistor. T. Durkop, et al., NANO LETTER, vol. 4, p. 35-39 (2004) reports performance of a field effect transistor (FET) produced by using one SWNT. According to the measurement results reported in the document, the mobility is 105 cm2/Vs, which outstrips a silicon transistor. However, the production process thereof has such a problem that the process includes such a step that electrodes are arranged on each of SWNT, which is a nanometer scale material, and thus it is not suitable for industrial use through mass production.
One approach for solving the problem includes a method of handling carbon nanotubes in a bulk state rather than each of carbon nanotubes. In particular, such a method is being variously investigated that carbon nanotubes, which is originally difficult to process due to the powder form thereof, are formed into a film to facilitate handling and processing thereof.
For example, JP-A-2000-109308 discloses a method for obtaining a carbon nanotube film by converting silicon carbide grown on a silicon wafer to carbon nanotubes through a heat treatment at a high temperature. However, the method requires that the substrate is heated to 1,700° C., and cannot be applied to such a substrate as plastics.
JP-A-2002-226209 discloses such a method for obtaining a carbon nanotube film by solubilizing carbon nanotubes through introduction of amide groups thereinto, and then accumulating them on a substrate through the Langmuir-Blodgett (LB) method. However, it is necessary in the method that functional groups are introduced into carbon nanotube for forming carbon nanotubes into a film. In other words, the method has such a problem that carbon nanotube having no functional group introduced cannot be formed into a thin film.
As a method for forming carbon nanotubes into a thin film without introduction of functional groups, it has been known that carbon nanotubes dispersed in a solvent are sprayed on a substrate. In this method, however, carbon nanotubes cannot be well dispersed due to aggregation thereof, and thus a uniform film cannot be formed. Therefore, such a solvent has been demanded that can well disperse carbon nanotubes for obtaining a uniform thin film.
As a solvent satisfying the demand, there has been the ionic liquid proposed in JP-A-2004-142972. JP-A-2004-142972 discloses that in the case where carbon nanotubes are mixed with the ionic liquid and segmentalized by applying a shearing force, they turn into a gelled state. However, there is no specific measure for forming a carbon nanotube thin film disclosed in the document.